1. Field of the Invention
This invention relates to a glazing panel which consists of or includes a pane comprising a pair of vitreous sheets adherent to an intervening layer of polymeric material.
The present invention has especial reference to the acoustic properties of such composite glazing panels, and in particular to the acoustic attenuation which may be afforded by such panels.
2. Description of the Related Art
According to the current theories, the acoustic attenuation afforded across a monolithic sheet depends on its mass per unit area, that is, for a given material, on its thickness. Briefly stated, the acoustic attenuation afforded over a particular range of sound frequencies is directly proportional to the thickness of the sheet Matters are complicated, however, by the fact that for any sheet, there will be two sound transmission peaks, one at each end of that range, and the frequencies at which these peaks occur are also determined at least in part by the thickness of the sheet. The lower frequency peak, the resonance peak, occurs at a frequency which depends on the shape and area of the sheet but also it increases in proportion to any increase in thickness of the sheet. The higher frequency peak occurs at the critical frequency of coincidence of the sheet, that is the frequency of free bending waves in the sheet, and that decreases in proportion to any increase in thickness of the sheet. By way of example, a 1 meter square monolithic panel of glass 4 mm thick has a resonance frequency of about 20 Hz and a critical frequency of coincidence (.phi..sub.m) of about 3000 Hz. A monolithic 8 mm sheet of glass of the same shape and area has a resonance frequency of about 40 Hz and a critical frequency of coincidence (.phi..sub.m) of about 1500 Hz. Theory predicts that doubling the sheet thickness will give an increase in sound attenuation (in fact of about 6 dB) for sound of a given frequency, but that, due to resonance and coincidence effects, this increase in attenuation will be masked, in the example given, at frequencies around 40 Hz and 1500 Hz. In fact, at its coincidence frequency of around 1500 Hz, an 8 mm glass sheet may well afford less attenuation than a 4 mm sheet. A particular problem is that by increasing sheet thickness in this way, a sound transmission peak is transferred into a region of the sound spectrum to which the human ear is more sensitive.
References to acoustic properties of panels throughout this specification are references to such properties measured in accordance with the German Federal Republic Standard VDI 2.719.
The position is more complicated when considering laminated glazing panels because of the different natures of the intervening layers of adhesive material and of the glass. However in a typical safety laminate there must be a rather strong bond between the glass and the adhesive so that in the event of breakage fragments of the glass are retained by the panel and do not fly off and possibly cause injury. In fact the acoustic behaviour of typical laminated safety glazings is scarcely distinguishable from the acoustic behaviour of a single monolithic sheet of glass of the same shape and area as the laminate and of the same thickness as the total thickness of the glass in the laminate. Thus for the purposes of this specification and as regards their acoustic properties, such conventional laminates are considered as equivalent to a single monolithic sheet.